1. Field of the Invention
The present invention relates to exhaust emission reduction systems for the removal of particulate matter and reactive nitrogen oxides (NOx) from diesel engine exhaust streams, and, more particularly, to improving the efficiency of emission reduction systems having a selective catalytic reduction component.
2. Description of the Related Art
Diesel engine combustion exhausts include carbon dioxide, carbon monoxide, unburned hydrocarbons, NOx, and particulate matter (PM). Increasingly, environmental regulations call for emissions controls to progressively lower diesel exhaust emission levels for NOx and PM. For example, EURO 4 (2005) and EURO 5 (2008) and U.S. 2004 and U.S. 2007 emissions limit standards. Regulations are increasingly limiting the amount of NOx that can be emitted during a specified drive cycle, such as an FTP (Federal Test Procedure) in the United States or an MVEG (Mobile Vehicle Emission Group) in Europe.
One of the ways known in the art to remove NOx from diesel engine exhaust gas is by catalyst reduction. A catalyst reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Two types of catalytic reduction are nonselective catalyst reduction (NSCR) and selective catalyst reduction (SCR). This invention relates to emission reduction systems including the SCR type of catalytic reduction.
Roth et al., U.S. Pat. No. 6,446,430, discloses a method and apparatus for reducing transient and steady-state NOx emissions in the exhaust gases of a vehicle powered by a diesel-fueled internal combustion engine which includes a reducing catalytic converter downstream of the engine. The catalytic converter includes a reducing catalyst and a system for injecting fuel oil as hydrocarbon (HC) reductant into the exhaust gas upstream of the catalytic converter. Roth recognizes that transient engine conditions will increase the temperature of the exhaust gas which, in turn, will raise the temperature of the catalytic converter to the point where the temperature window in which NOx conversion occurs may be exceeded.
Conversion efficiency of NOx catalysts is temperature dependent. The efficient operation temperature range is generally between 150 and 500° C., depending on the catalyst. Above 750 to 800° C. catalysts may be damaged. During engine operations involving high loads, exhaust gas temperatures may exceed these ranges.
Diesel particulate filters (DPF) for the removal of PM from a diesel engine exhaust stream have been proven to be extremely efficient at removing carbon soot. A widely used DPF is the wall flow filter which filters the diesel exhaust by capturing the particulate material on the porous walls of the filter body. Cutler et al., U.S. Pat. No. 6,464,744, discloses a porous ceramic diesel exhaust particulate filter. The ceramic filter includes a plurality of end-plugged honeycomb structures which in combination act to trap and combust diesel exhaust particulates. As particulate material collects, eventually the pressure drop across the filter rises to create back pressure against the engine and regeneration of the filter becomes necessary. The regeneration process involves heating the filter to initiate combustion of the carbon soot. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400 to 600° C. to a maximum of about 800 to 1,000° C.
In currently available systems, there is a problem of effective combustion of diesel PM at exhaust stream temperatures of 300° C. or below. While the temperature of diesel exhaust stream may exceed 500° C., it is generally considerably lower, e.g., 300° C. or below, especially under a low engine load condition, and, as noted above, catalytic filters are not particularly effective for combusting particulate at such low temperatures. In addition, carbon combustion by thermal means requires temperatures of up to 600° C.
While the above systems have been found beneficial in reducing certain diesel exhaust emissions, it has also been found beneficial if such systems are operated at temperatures that maximize their efficiency. Specifically, DPF regeneration is accomplished at a temperature above a particular threshold, often a temperature higher than typical diesel exhaust stream temperatures, and NOx catalysts, such as SCR, operate most efficiently in a temperature window of typically 300° C. to 450° C.